1. Field of the Invention
The present invention relates to a solid-state laser generator with intracavity frequency conversion, and more specifically relates to a solid-state laser generator that converts frequency by using a nonlinear optical crystal.
2. Description of the Related Art
Nd:YAG lasers or other solid-state laser generators are widely used as machining lasers. Recently, solid-state laser generators are rapidly increasing in output from several hundred watts to several kilowatts, and the field of application of the lasers is accordingly expanding from the conventional microfabrication field to welding and cutting applications in the automotive industry. Nevertheless, most solid-state laser generators have an emission wavelength range that is limited to the near infrared range of about 1 μm, bringing about a drawback in that reflectivity with respect to wavelengths outside the near infrared range is high, and the machining efficiency is poor in copper, silicon, and some other materials that have a low absorption rate.
For this reason, methods have been proposed in the prior art (Japanese Patent Application Kokai Publication No. 6-5962, Japanese Patent Application Kokai Publication No. 6-21553, Japanese Patent No. 3197820) in which LiB3O5 (lithium triborate, LBO), KTiOPO4 (KTP), β-BaB2O4 (barium borate, BBO), and other nonlinear optical crystals are used to convert the emission wavelength to a second harmonic, reduce the reflectivity in the surface of the workpiece, increase laser light absorption, and thereby increase the machining efficiency. External cavity frequency conversion and intracavity frequency conversion are types of frequency conversion in which such nonlinear optical crystals are used.
External cavity frequency conversion has a low conversion rate from a laser light at a fundamental frequency (hereinafter referred to as fundamental laser light) to second-harmonic laser light, and fundamental laser light must be condensed with high power density in the nonlinear optical crystals in order to output high-output laser light. For this reason, the upper limit of the output obtained from a simple resonator is in the over 100 W category, and it is difficult to achieve a higher output when reliability is considered.
Intracavity frequency conversion has a high conversion rate to second-harmonic laser light and better reliability in comparison with external cavity frequency conversion. This method is disadvantageous, however, in that the thermal lens effect generated in the solid-state laser medium does not allow higher output to be obtained while a stable resonant condition is maintained. The thermal lens effect is a phenomenon in which the solid-state laser medium is heated by being excited, the temperature distribution produced inside the solid-state laser medium creates a refractive index distribution, and the solid-state laser medium behaves like a lens.
Common solid-state laser media absorb almost no fundamental laser light, but frequency-converted laser light often has high absorption characteristics, particularly with respect to laser light that has been converted to a higher frequency. In view of the above, solid-state laser generators that adopt intracavity frequency conversion are configured to bend the optical path of the laser light at least once and separate the optical path into frequency-converted laser light and fundamental laser light by using a dielectric multilayer film mirror to bring out only the frequency-converted laser light from the resonator, and thereby prevent the frequency-converted laser light from being absorbed by the solid-state laser medium and obtain frequency-converted laser light having good efficiency.
FIG. 1 is a diagram showing the configuration of a resonator in a conventional solid-state laser generator with intracavity frequency conversion. The resonator 100 of a conventional solid-state laser generator has a configuration in which a flat mirror 104, a Q-switch 103a, a solid-state laser medium 101, a Q-switch 103b, and a flat mirror 107 are disposed in sequence in a single row, as shown in FIG. 1. The flat mirror 107 is disposed so that the laser light transmitted through the Q-switch 103b enters at a 45° angle. A concave mirror 110 and an LBO or another nonlinear optical crystal 102 for converting the frequency of the laser light are disposed in sequence in the direction of travel of the laser light reflected by the flat mirror 107, more specifically, in a direction that is 90° to the incident direction of the laser light.
The flat mirror 104 of the resonator 100 is provided with a dielectric multilayer high-reflection film that reflects the perpendicularly incident fundamental laser light 109, on the surface that faces the Q-switch 103a, more specifically, the laser light incident surface. The flat mirror 107 is a mirror for separating the converted laser light 108, and the laser light incident surface of the mirror is provided with a dielectric multilayer film whereby the fundamental laser light 109 incident at 45° is reflected with high reflectivity, and the second-harmonic laser light 108 incident at 45° is transmitted. The concave mirror 110 is provided with a dielectric multilayer high-reflection film that reflects both the perpendicularly incident fundamental laser light 109 and second-harmonic laser light 108.
For this reason, the fundamental laser light 109 generated in the solid-state laser medium 101 is reflected by the flat mirror 104, is then passed through the solid-state laser medium 101 again, is reflected by the flat mirror 107, and is allowed to enter the nonlinear optical crystal 102. A portion of the fundamental laser light 109 is converted to second-harmonic laser light 108 by the nonlinear optical crystal 102. The fundamental laser light 109 and second-harmonic laser light 108 that have passed through the nonlinear optical crystal 102 are reflected by the concave mirror 110, transmitted through the nonlinear optical crystal 102 again, and converted to another frequency. The fundamental laser light 109 is then reflected by the flat mirror 107 and directed to the Q-switch 103b, and the second-harmonic laser light 108 is transmitted through the flat mirror 107 and brought outside the system. The second-harmonic laser light 108 alone is thereby brought outside the system, and the fundamental laser light 109 is confined therein. In the conventional solid-state laser generator, there is an asymmetry with respect to the solid-state laser medium 101 that is proportionate to the space occupied by the nonlinear optical crystal 102 and the flat mirror 107, at least one mirror is configured as a concave mirror 110, rather than all the mirrors disposed therein being configured as flat mirrors.
The above-described prior art, however, has the drawbacks described below. As described above, a solid-state laser generator with intracavity frequency conversion has a high conversion rate from the fundamental laser light to a second-harmonic laser light, but because a thermal lens effect is created in the solid-state laser medium, the thermal lens focal distance of the solid-state laser medium is reduced when the intensity of the excitation light emitted to the solid-state laser medium is increased in order to obtain high-output laser light.
In a conventional solid-state laser generator with intracavity frequency conversion, the distance between the two flat mirrors, that is to say, the resonator length must be kept to a magnitude of four or less of the focal distance of the solid-state laser medium in the case that two flat mirrors are disposed, for example, as resonator mirrors on both sides of the solid-state laser generator in order obtain a stable resonance state. The shorter the resonator length is, the more stable the high laser output is. However, when the optical path of laser light is configured so as to be folded back, as in the resonator 100 shown in FIG. 1, space is required not only for a nonlinear optical crystal 102, but also for a flat mirror 107 that folds the optical path by reflecting the fundamental laser light 109 and transmits the second-harmonic laser light 108 to separate this light from the fundamental laser light 109. Thus, a solid-state laser generator with a conventional resonator configuration in which the optical path is folded back has drawbacks in that the resonator length cannot be made shorter and a higher output is difficult to obtain.
Another drawback is that it is difficult to increase the number of solid-state laser media disposed inside the resonator in a conventional solid-state laser generator with intracavity frequency conversion. FIGS. 2A to 2C are diagrams that show the propagation state of the fundamental laser light when the components of the resonator 100 shown in FIG. 1 are rearranged so as to form a linear optical path. FIG. 2A shows the case in which the intensity of the excitation light input to the solid-state laser medium has low intensity. FIG. 2B shows the case in which the intensity of the excitation light is an intermediate intensity. FIG. 2C shows the case in which the intensity of the excitation light is high. The propagation state of the fundamental laser light 119 shown in FIGS. 2A to 2C is separately obtained by calculation. Shown is a transverse mode of propagation of the fundamental laser light 119 confined within the resonator 100. Two solid-state laser media 111a and 111b are disposed between a flat mirror 114 and concave mirror 120 placed facing each other inside the resonator, as shown in FIGS. 2A to 2C, and the interval between the solid-state laser medium 111b and the concave mirror 120 is extended by a distance equal to the space occupied by the nonlinear optical crystal 112. Thus, when the configuration of the resonator with respect to the solid-state laser medium is asymmetric, the propagation mode formed in this case is also an asymmetric transverse propagation mode. For this reason, the number of solid-state laser media cannot be easily increased inside the resonator in a conventional solid-state laser generator with intracavity frequency conversion.